As a mounting structure for use in electronic equipment, there is heretofore known a structure in which an electronic component is mounted on a wiring board.
For example, in Japanese Unexamined Patent Publication JP-A 2011-187473, there is disclosed a wiring board comprising an electrically-conductive layer and a resin layer which covers the electrically-conductive layer. The resin layer includes a resin portion and inorganic insulating particles dispersed in the resin portion, and is in contact with one main surface and side surfaces of the electrically-conductive layer.
Since the inorganic insulating particle and the resin portion differ from each other in material characteristics, it follows that the adhesion strength between the inorganic insulating particle and the resin portion is low. Meanwhile, when reflow soldering is carried out for the installation of an electronic component on the wiring board, the wiring board is subjected to heat. At this time, concentration of thermal stress resulting from the difference in thermal expansion coefficient between the electrically-conductive layer and the resin layer tends to occur in the vicinity of a corner between one main surface and the side surface of the electrically-conductive layer. In consequence, a crack may be developed from the corner toward the adherent surfaces of the resin portion and the inorganic insulating particle bonded to each other with low adhesion strength. This cracking is prone to occur especially when a content rate of the inorganic insulating particles is increased in order to lower the thermal expansion coefficient of the wiring board with the aim of improving the reliability of connection between the wiring board and the electronic component.
If such a crack appears in the resin layer, when a voltage is applied to the electrically-conductive layer, part of the electrically-conductive layer ionized by the voltage will find its way into the crack, with the result that the adjacent electrically-conductive layers are susceptible to electrical short-circuiting (ion migration). This leads to deterioration of the electrical reliability of the wiring board.